Firefox does not handle filenames with spaces . When a user clicks on an attachment with spaces, the filename is truncated to the first whitespace. While IE & Safari both handle this, Firefox refuses to accept mime headers with unquoted filename parameters. According to Firefox's bugzilla/knowledgebase, Firefox's behavior is the correct behavior and it's a problem with most webservers or web applications. This problem can be easily corrected by surrounding the filename parameter with double quotes.